FIG. 9 is a side view conceptually showing an example of the support structure between the motor and the housing of a centrifugal separator according to the related art, the centrifugal separator having a mechanism to rotate a rotor with the motor.
In the centrifugal separator, a rotational shaft 1a of a motor 1 is coupled to a rotor 2, and the rotor 2 is provided with sample mounting parts 2a. The bottom part of the motor 1 is anchored to a flange 3 or integrally formed with the flange 3.
FIG. 10 is a sectional view taken along line 10-10 in FIG. 9 in the direction of the arrows. The flange 3 and a housing 4 are coupled to each other by screws 6 via elastic members 5. Herein, in order for the housing 4 to receive a reaction occurring when the motor 1 rotates and drives the rotor 2, the screws 6 are arranged at even intervals, for example, in sets of four, six, or eight (six in the illustrated example) on the circumference about the motor 1, as shown in FIG. 10, such that the motor 1 is anchored to and supported by the housing 4 evenly in the peripheral direction.
In a device such as a centrifugal separator and a washing machine in which a rotor is rotated with a motor, there is a case where the substantial center of gravity of the rotor is eccentric relative to the axial center of the rotational shaft of the rotor due to an unbalanced distribution of masses on the rotor. In such a case, the rotor makes so-called oscillating motion relative to the rotational shaft of the motor and, in certain conditions, may end up hitting part of a housing, thereby causing a breakage accident.
For the purpose of reducing the oscillating motion, a dummy mass is arranged such that the distribution of the masses becomes symmetrical about the rotational shaft of the rotor so as to compensate for the unbalanced distribution of the masses on the rotor. However, since this method per se could be insufficient to obtain the intended results, a ball balancer is sometimes attached as a further countermeasure. The centrifugal separator shown in FIG. 9 is an example in which a centrifugal separator has the ball balancer 7, and FIG. 11 is a sectional view taken along line 11-11 in FIG. 9 in the direction of the arrows.
The ball balancer 7 has an annular path 7a and balls 7b, e.g. metal balls, that are placed in the annular path 7a such that the balls 7b do not fill up the annular path 7a completely but are freely movable therein, and the ball balancer 7 is arranged so that central lines of the rotational shaft of the rotor 2 and the annular path 7a coincide with each other. When the rotor 2 rotates, the annular path 7a of the ball balancer 7 also rotates. Then, due to friction caused between the annular path 7a and the balls 7b, the balls 7b also rotate in the annular path 7a along with the rotation of the rotor 2.
When the rotor 2 is eccentric, i.e., when the substantial center of gravity W of the rotor 2 with samples mounted thereon does not coincide with a rotational axis O of the rotor 2, the rotor 2 performs the oscillating motion about the rotational axis. At this time, the ball balancer 7 is caused to rotate about the rotational axis deviating from the central line of the annular path 7a, whereby friction occurs between the inner wall of the annular path 7a and the balls 7b and the balls 7b are subject to complex actions. It is known that when the ball balancer 7 rotates during the oscillating motion, the balls 7b make up an arrangement such as to reduce the oscillating motion.
FIG. 11 schematically shows an example of the arrangement of the balls 7b in the ball balancer 7 when the ball balancer 7 is functioning properly. As seen in a coordinate system rotating with the rotation of the rotating rotor 2, when the substantial center of gravity W of the rotor 2 is eccentric from the central line O of the annular path 7a of the ball balancer 7, the balls 7b move to appropriate positions on a side opposite to the substantial center of gravity W across a central line O and placed in arrangement such as to make the substantial center of gravity W coincide with the central line O of the annular path 7a (hereinafter the arrangement will be called “balanced arrangement”). According to this arrangement, the substantial eccentricity of the system including the rotor 2 and the ball balancer 7 is offset, resulting in the effect of reducing the oscillating motion.
As shown in FIG. 12, the path 7a of the ball balancer 7 may be divided into a plurality of parts by a plurality of (four, for example) partitions 7c. In this case, the balls 7b of the ball balancer 7 are only allowed to move within the respective parts. This configuration is designed for cases including stoppage of the rotor, aiming at preventing the balls 7b from continuing to rotate in the annular path 7a due to inertia from the stoppage. Even if the annular path 7a is partitioned as described above, it is known that the balls 7b move in a direction opposite to a direction in which the substantial center of gravity W of the rotor is eccentric relative to the rotation center O of the rotor 2, so that the balls 7b make up an arrangement (balanced arrangement) to cancel out the effect of the eccentricity during rotation, similarly to the case in which the annular path 7a is not partitioned as shown in FIG. 11. However, since the balls 7b are not allowed to move across the partitions, the balls 7b are placed in arrangement according to which the balls 7b cancel out the eccentricity of the substantial center of gravity W within the respective partitions, as shown in FIG. 12, for example.
PTL 1 discloses a technology by which a centrifugal separator using the above ball balancer is allowed to smoothly operate without excessively increasing the vibration of a rotor when resonance of the rotor occurs, aims to increase a correction amount of an unbalanced mass at high-speed rotation and reduce the vibration of the rotor, and has the cross section of the annular path of the ball balancer formed in a prescribed shape.